It sounds like a LOT of things changed on him all at once, which is never good.
What happens if you cut back on pellets? Just because something is good on paper, doesn't mean it's good in practice. Why did you change from SmartGut to SmartGut Ultra?
What caused him to go on gastroguard (which made our thoroughbred mare a FREAK)
You need to have answers for these questions, and make sure you are not overfeeding and then your horse isn't able to relieve any of the excess energy.
Check all avenues. Run a bloodpanel, too.